Large trochids from the Eocene of Southern Patagonia

MIGUEL GRIFFIN Y MARTÍN RODRIGUEZ RAISING

Valdivia - Chile

Congreso; VII Congreso Latinoamericano de Malacología; 2008

The Man Aike Formation is a unit recognized in subsurface and expsures in southwestern Santa Cruz, within the Magallanes basin. It includes sandstones and conglomerates with intercalated shell-beds. At the section studied in Estancia 25 de Mayo this unit is 80 meters thick and constitutes part of the infilling of an incised valley.  Based on microfossils, the age of this unit is believed to be middle to late Eocene. At this locality the section bears a 40 cm thick shell deposit that can be laterally followed for a distance of about 300 meters. It carries also dispersed pebbles, with a maximum diameter of 5 cm, within a moderately sorted sandy matrix. It is interpreted as a part of a ravinement surface with concentration of shells and coarser material and winnowing of finer particles. It contains a rich fauna of molluscs – mostly bivalves – including Venericardia, “Ostrea”, Lopha, Lahillia, among others. The only gastropods reported from this bed are Turritella sp., Fagnanoa sp., and two fragments of a large shell inaccurately referred to Pleurotomaria. A large almost complete specimen recently collected suggests that it belongs in an entirely different group, more closely related to species described from younger beds in southern Chile and Patagonia, and probably related to Astele. The remarkably large shell measures at least 250 mm and 220 mm wide. The apex is missing, as is most of the shell material, but the little shell preserved shows an apparently smooth surface, with flat whorls and adpressed sutures, in all similar to the shells of the younger Oligocene and Miocene Patagonian species.